Caption: Leguminous underground stem. Close-up of the rhizome (underground stem) of the leguminous plant Thermopsis montana. The rhizome or rootstock (thick, running from top to bottom) is a fleshy food storage organ that grows horizontally under the ground. Growing from the rhizome are shoot buds (thick, white) by which the plant spreads vegetatively. The thinner brown and white structures are the roots, on which are small nodules (round). These nodules are a common feature of leguminous plants. They contain Rhizobium bacteria which convert atmospheric nitrogen into a form that the plants can use. Photographed in Western USA.